The present invention relates to a method of and an apparatus for treating radioactive liquid wastes containing surface active agents and, more in particular, it relates to a method of and an apparatus for treating radioactive liquid wastes containing surface active agents suitable for treatment of radioactive laundry liquid wastes generated from nuclear power plants.
In nuclear power plants, radioactive liquid wastes containing surface active agents such as liquid wastes after laundry of working clothes and toilet liquid wastes (hereinafter referred to as laundry liquid wastes). The laundry liquid wastes are subjected to removal or oxidative decomposing treatment of organic substances such as surface active agents contained therein, removed with radioactive cruds (hereinafter referred to as SS components) and then discharged out of a facility. Known methods of treating laundry liquid wastes include a method of adsorbing organic substances with activated carbon followed by filtration (hereinafter referred to as an activated carbon filtration treatment), a method of aerating oxidative gases such as ozone or oxygen to the laundry liquid wastes, a method of decomposing organic substances by addition of an aqueous oxidative solution such as aqueous hydrogen peroxide to the laundry liquid wastes (hereinafter referred to as an oxidizer treatment) or a method of decomposing treatment by irradiation with ultraviolet rays to the laundry liquid wastes after aeration or addition of the oxidizer (hereinafter referred to as a UV treatment).
The activated carbon filtration treatment is excellent in that SS components can be removed upon filtration of the laundry liquid wastes. However, spent activated carbon forms secondary wastes, which requires a facility for treating the activated carbon. The oxidizer treatment has a merit capable of oxidatively decomposing precipitation components formed by bonding of sweat or dirt components from human bodies and surface active agents contained in the laundry liquid wastes (hereinafter referred to as organic precipitation components) in the same manner as for the dissolved components, as well as extremely reducing the amount of secondary wastes. However, since the oxidizing reaction proceeds slowly, the oxidizer treatment is not suitable for a case requiring a high liquid waste treating rate. The oxidizing reaction of organic substances is mainly the reaction of extracting hydrogen atoms from the organic substances caused by hydroxy radicals (hereinafter referred to as OH*) formed by reaction of dissolved oxidizer and water in the laundry liquid wastes. Therefore, the problem of the oxidizer treatment is usually caused in that the OH* forming rate is extremely low when only the oxidizer is used. Further, removal of SS components is necessary before or after the oxidizer treatment. Since the reaction between ultraviolet rays and the oxidizer can outstandingly improve the OH* forming rate, the UV treatment can improve the liquid waste treating rate by the oxidizing reaction and reduce the amount of secondary wastes to be produced. However, in the UV treatment, the decomposing performance is lowered for the treatment of liquid wastes containing a great amount of organic precipitation components or SS components since transmission of ultraviolet rays is deteriorated. Accordingly, SS components have to be removed, for example, by using filtration as a pretreatment. In this process, the filter clothes suffer from clogging due to reaction products of surface active agents and organic precipitation components such as dirt.
A decomposing treatment for organic substances not using the UV treatment is disclosed in xe2x80x9cNew Technology for Ozone Utilization, New editionxe2x80x9d (published from Sanshu Shobo, in 1993). xe2x80x9cNew Technology for Ozone Utilization, New editionxe2x80x9d discloses, on page 79, irradiation of ultraviolet rays, as well as addition of hydrogen peroxide, addition of aqueous alkali and use of a catalyst as a method of improving the OH* forming rate from dissolved ozone. It is described that the treating rate of the organic substances can be improved by about three times compared with the case of using only ozone, by appropriately controlling the addition amount of hydrogen peroxide, assuming that the ozone aeration amount is constant. Further, it is also described that the treating rate can be improved by increasing the ozone gas aeration amount and the ozone concentration in the aeration gas while keeping the ratio constant between the ozone gas aeration amount and the addition amount of hydrogen peroxide.
xe2x80x9cNew Technology for Ozone Utilization, New editionxe2x80x9d describes, on page 217, that a distribution coefficient (gas phase concentration/dissolved concentration) controlling the amount of ozone gas dissolved into water lowers remarkably along with the rise of temperature of the solution and it is reduced to zero at 60xc2x0 C. or higher. The OH* forming rate constant increases by the increase of the self decomposition of ozone along with the rise of temperature of the solution. However, since the effect of lowering the reaction rate caused by the decrease of the dissolution amount of ozone due to lowering of the distribution coefficient becomes predominant, the oxidizer treatment has been conducted so far under cooling rather than under heating (for example, refer to page 233 of this literature). While the decomposing treatment of organic substances at a low temperature is effective for the treatment of liquid wastes containing organic substances at a low concentration, it is not suitable for the treatment of liquid wastes containing organic substances at a high concentration such as in laundry liquid wastes in a short period of time.
An object of the present invention is to provide a method of and an apparatus for treating radioactive liquid wastes containing surface active agents that can increase the rate of treating organic substances by using ozone.
The first feature of the invention for attaining the foregoing object resides in adding hydrogen peroxide and charging ozone to radioactive liquid wastes containing organic substances including surface active agents in which ozone is charged while heating the radioactive liquid wastes to 50xc2x0 C. or higher.
Since the radioactive liquid wastes are heated to 50xc2x0 C. or higher, bonding reaction between ozone and hydrogen peroxide is increased, and the solubility of ozone into the radioactive liquid wastes is increased under the effect of an extremely high solubility of hydrogen peroxide. Due to the effect of increasing amount of ozone in the radioactive liquid wastes and the effect of hydrogen peroxide, the amount of OH* formed in the radioactive liquid wastes is increased, to increase the decomposing rate of organic substances including surface active agents contained in the radioactive liquid wastes. Therefore, the concentration of the organic substances contained in the radioactive liquid wastes is lowered in a short period of time. Occurrence of secondary wastes is remarkably suppressed.
A second feature of the present invention for attaining the foregoing object resides in charging ozone to a plurality of aeration vessels to which the radioactive liquid wastes are supplied and charging ozone once charged into one of the aeration vessels and discharged therefrom into another aeration vessel. By the provision of the plurality of aeration vessels, the height of each aeration vessel can be reduced to facilitate installation to a controlled area in a radioactive substance handling facility. Further, the amount of treating the radioactive liquid wastes containing the surface active agents per unit time can be increased by the provision of the plurality of aeration vessels, and the ozone utilization efficiency is increased.
A third feature of the present invention for attaining the foregoing object resides in breaking bubbles of ozone charged in the aeration vessel. Since the bubbles of ozone are broken, the bubbles can be made fine to increase the area of contact between ozone and the radioactive liquid wastes. This leads to increase of the amount of ozone dissolved in the radioactive liquid wastes to further increase the decomposing rate of organic substances contained in the radioactive liquid wastes.
A fourth feature of the present invention for attaining the foregoing object resides in addition of an alkali solution to radioactive liquid wastes. The amount of ozone dissolved into the radioactive liquid wastes is increased by the effect of the alkali solution. Therefore, the decomposing rate of organic substances can further be increased. The self decomposing rate of ozone in the radioactive liquid wastes can be remarkably increased by adjusting pH of the radioactive liquid wastes supplied to the aeration vessel preferably to 7 or higher. As a result, the amount of ozone absorbed to the boundary layer between the bulk layer of the radioactive liquid wastes and ozone bubbles is increased to increase the possibility of causing gas/liquid reaction. The amount of ozone absorbed per unit volume of the radioactive liquid wastes can be increased to thereby decrease the number of aeration vessels.